1. Field
The invention relates generally to wireless communication, and more specifically to broadcast/multicast services.
2. Background
Broadcast, or multicast services, refers to a communication system used to transmit information from a transmitter to multiple receivers or users. Examples of broadcast, or point-to-multipoint communication systems, include dispatch systems, such as used by police, trucking companies, and taxi companies where a central dispatcher broadcasts signals to one or more vehicles. A broadcast signal may be directed to a specific vehicle or to all vehicles simultaneously.
As mobile radio networks have become commonplace, such as cellular telephone networks, customers have begun to desire to receive broadcast of multimedia, such as video and teleconferencing, using Internet Protocol (IP) over a wireless communication link. For example, customers desire to be able to receive streaming video, such as television broadcast, on their cell phone or other portable wireless communication device. Other examples of the type of data that customers desire to receive with their wireless communication device include multimedia broadcast and Internet access.
A typical wireless communication channel has limited bandwidth and at times may experience significant error rates. Various techniques for transmitting messages according to broadcast and multicast services (BCMCS) have been developed. In general, these techniques include formatting the message data into packets with a header that includes information about the data within the packet. In BCMCS communications, a content provider, or content server, generates a data stream to be broadcast to multiple receivers, or users. The data stream is converted to data packets to make up a BCMCS data stream that is then broadcast to multiple communication devices simultaneously.
It may be desired that only some WCDs receive BCMCS streams. For example, a content provider may desire that only authorized WCDs, such as ones that have paid a subscription fee, be able to receive the content. Because the BCMCS streams are broadcast through the air, and therefore may be received by both authorized and unauthorized WCDs, different ways to secure the BCMCS streams have been developed.
A protocol for secure transmission of data, including BCMCS streams, that has been developed by the Third Generation Partnership Project Two (3GPP2) is known as the Secure Real-time Transport Protocol (SRTP). In a SRTP broadcast, session keys that are used to de-crypt the BCMCS stream are generated from a master key, packet index (PI), and other key generation materials. Because the session key is used to decrypt the broadcast stream, periodically during a SRTP broadcast the session key is updated to a new value to prevent unauthorized WCDs from receiving the content. Updating of the session key requires coordination between the provider of the broadcast stream and the authorized WCDs. In addition, increasing the randomness of the updated session key is desirable, thereby making it more difficult for an unauthorized WCD to determine what the new session new is.
There is therefore a need in the art for improving the updating of the session key in a SRTP BCMCS session.